The human brain contains more than 100 billion cells, the majority being non-excitable glial cells. Recent studies, including those from the applicant's lab, demonstrate that glial cells of vertebrate and invertebrate nervous systems have remarkably dynamic roles in the regulation of physiological and behavioral processes. Studies in mammals have demonstrated that neurons and glia communicate with one another and this has given rise to a model of the tripartite synapse wherein a glial cell (an astrocyte) cooperates with presynaptic and postsynaptic neuronal elements to regulate communication events and behavioral processes (see Significance). Recent studies from the applicant's lab describe a role for a defined population of Drosophila astrocytes in the regulation of circadian behavior. Other studies have documented additional functions for fly glia in the regulation of neurotransmission and behavior (reviewed in Jackson and Haydon, 2008). In the present application, we propose experiments to elucidate the functions of glia in circadian timing. Our studies will employ Drosophila so as to be able to utilize sophisticated genetic techniques to study neuron-glia interactions in the circadian system. The work will utilize innovative genetic, behavioral, imaging and electrophysiological approaches and, importantly, the PI and co-I have complementary strengths in these areas. We propose three specific aims that will test explicit hypotheses about neuron-glia interactions in the circadian system: (1) Test the hypothesis that gliotransmission or other glial processes are essential for circadian behavior; (2) Test the hypothesis that glia regulate pacemaker neurons; and (3) Test the hypothesis that clock neurons regulate glial rhythms. We expect that the results of these studies will highlight general mechanisms by which neurons and glia cooperate to influence circadian rhythmicity and other behaviors. In most neurological disorders and psychiatric states, glial cell gene expression profiles are altered, and it is likely that this initiates dramatic structural/functional changes in the brain that lead to these disorders. Alterations of glial cell biology have been implicated in mental and neurodegenerative diseases including multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS), schizophrenia, epilepsy, and Alzheimer's. Our proposed studies of glia and circadian control mechanisms have considerable significance for an understanding of pathophysiological conditions such as jetlag and sleep/wake disorders resulting from environmental or genetic perturbations of the circadian system. Molecular components of the circadian system are conserved between insects and mammals, including humans, and Drosophila is an outstanding model for conducting genetic investigations of circadian behavior. It is anticipated that the results of our proposed studies will provide important and general insights about the interaction of glia with the neuronal circuitry controlling behavior, insights which are critical for understanding the roles of glial cells in health and disease.